Colon cancer is a disease in which abnormal cells begin growing in the inner lining of the large intestine and form a tumor that can grow through the bowel wall and spread to nearby lymph nodes or other organs.

Early colon cancer may cause subtle changes such as a lasting change in bowel habits, unexplained tiredness from anemia, mild abdominal discomfort or bloating, and loss of weight or appetite; as the disease progresses patients may notice visible blood in the stool, dark stools, persistent abdominal pain or a sensation of incomplete bowel emptying.

Risk increases with age and with a personal or family history of colorectal cancer, adenomatous polyps, or inherited syndromes; long standing inflammatory bowel disease, a diet high in red and processed meat and low in fiber, obesity, smoking, and a sedentary lifestyle also raise the risk.

Diagnosis starts with a doctor taking a medical history and performing a physical examination followed by a colonoscopy with biopsy to confirm the diagnosis; blood tests and imaging studies such as CT scans are then used to determine how far the cancer has spread.

For localized disease the main curative treatment is surgery to remove the tumor and nearby lymph nodes often followed by chemotherapy when indicated; advanced or metastatic disease is treated with systemic chemotherapy, targeted therapies, immunotherapy when appropriate, and selective local treatments to liver or lung metastases in suitable patients.

The outlook depends largely on the stage at diagnosis with early detection offering the best chance of cure; timely treatment significantly improves survival and long term outcomes.

After treatment patients typically undergo regular follow up with clinical examinations, periodic blood tests and scans when needed, and surveillance colonoscopies to detect recurrence or new polyps at an early, treatable stage.